Ultraviolet Radiation and the Ozone Layer

by: Bartlomiej Kleczynski

written as part of a project for Earth Science 002: Gaia - Our Earth System at the Pennsylvania State University.

The Ozone Layer is the part of our atmosphere located in the stratosphere, over 10 kilometers above the surface of Earth. It is comprised mostly of Ozone, a colorless gas, with the chemical formula of (O₃). Ozone is responsible for breaking down the harmful rays of Ultraviolet Radiation coming from our Sun. These rays can be classified into three different types, namely UV-A, UV-B, and UV-C. UV-C is the portion of Ultraviolet Radiation with the shortest wavelength, and therefore, the highest energy. Rays of UV-C radiation are extremely harmful to living things on our planet. Fortunately, the Ozone Layer and regular Oxygen - (O₂) ) completely breaks down most of the rays before they can reach the ground. However, regular Oxygen only works well against this particular type of UV Radiation. Ozone, on the other hand, works well to absorb the other, less dangerous band of UV Radiation, UV-B. UV-A is not very well absorbed by any of these elements, however, it is not a very harmful type of radiation.

The chart below shows the strong inverse correlation between the presence of Ozone and the amount of UV-B radiation which reaches the ground. If you click on the arrows to move forward through the presentation, you can notice that as the ozone level decreases, the rate of observed UV-B radiation that reaches the Earth's surface increases proportionally. ss₃₂asasd

As you can see, the Ozone Layer is very important to our protection from over-exposure to the harmful rays of Ultraviolet Radiation, especially those of UV-B band. Without the Ozone Layer, no other element would be as successful in destroying the radiation, and it would reach us to cause problems.

We will now look at how the Ozone in the Ozone Layer completes the task of absorbing the UV Radiation, and how the radiation is necessary for the Ozone cycle. The following chart displays the complete cycle through which Ozone goes from creation to destruction.

If you follow the path from beginning to end, the first thing that happens is that a UV photon hits an Oxygen (O₂) molecule, splitting it into two single Oxygen (O) molecules. The energy of the UV photon is transferred into heat.

The two Oxygen (O) molecules then go on and eventually collide with Dioxide (O₂) molecules. Through an effect similar to a "pool-ball hit", a third, undefined molecule M acts as a platform for the Dioxide molecules. When the single Oxygen molecules collide with the Dioxides, they stick, because the energy of the collision is transferred onto the M molecule. The resulting two molecules are those of Ozone (O₃). That is how Ozone is created.

At some point, a new UV Photon will come along, and collide with the Ozone molecule. The result is that the Ozone (O₃) molecule will break up into a Dioxide (O₂) and an Oxygen (O) atom. Again, the energy of the photon will be changed into heat. In fact, so much heat is released in these reactions, that the stratosphere is warmed to temperatures just below those of the Troposphere.

The Dioxide molecule will eventually be reused to once again form a new Ozone molecule. The single Oxygen atom will over time collide with an Ozone molecule, and split it, forming two Dioxide atoms, which can be used to start over the whole process and form more Ozone molecules.

As you can see from the diagram above, the whole process remains in a very stable equilibrium, all of the molecules are used and reused, recycled throughout. However, humans have introduced new compounds and therefore, new molecules into the atmosphere, some of which have detrimental effects on the atmosphere. Molecules such as Chlorine offset the cycle, destroy hundreds of thousands of Ozone molecules, and create problems such as the present Ozone Hole over Antarctica.

Resources

1. University of Wisconsin - Chemical of the week - Ozone

http://scifun.chem.wisc.edu/chemweek/ozone/ozone.html

2. Sparling, Brien - NASA NAS - "The Ozone Layer"

http://www.nas.nasa.gov/Services/Education/Resources/TeacherWork/Ozone/Ozone.layer.html

3. US Environmental Protection - Stratospheric Protection Division - "Myth: No Link Exists Between Ozone Depletion and Higher UV Levels"

http://www.epa.gov/ozone/science/ozone_uv.html

4. National Science Foundation - Global Change Research Programs - "Polar Ozone Depletion / UV Radiation Effects"

http://www.nsf.gov/geo/egch/gc_stratoz.html

Page Last Updated:

12/06/00